LeAnn Rimes Defends Eddie Cibrian Railing

Fairly notoriously, too. Her affair with Eddie Cibrian ended both her marriage and his, to Brandi Glanville, last year. So why did she do it? She tells Shape:

"My relationship with Dean was great, but ultimately it wasn't fulfilling. We got married so young: I was 19, he was 21; as we got older, we grew apart."

Basically, kids, don't get married at 19.

Last year, rumors of LeAnn's on-set fling with Eddie Cibrian while shooting a Lifetime movie in late 2008 surfaced. They denied the story, but were totally busted.

Rimes - who now lives with Cibrian and his two young sons - isn't proud of her behavior: "I understand why people are disappointed in me," the singer admits.

"Especially since I grew up as America's sweetheart ... I think that any relationship is hard to get out of, and I certainly don't think the way I did it was right."

She still feels guilt. The controversial star says after her fans learned of her affair she got "so depressed." Adds Rimes, "I didn't want to get out of bed."

"I truly believe there are lessons in it. Cultivating strength from rough situations is the most important. After going through this, I can face anything."

She also credits exercise with helping her snap out of it ... and she may need to stay in shape for the next time Brandi Glanville gets all up in her grill.

"Just getting to the gym - even for a few minutes - was a big accomplishment," says Rimes. "It gave me a bit of sanity. Even now, on days when I'm really frustrated, I'll walk into the gym and say to my trainer, 'We're boxing today. I need to hit things.'"

She and Cibrian now go to the same gym. Cute.

"Sometimes we take group-cycling classes together, but when it comes to weights, we pretty much do our own thing," she says. "He's going to kill me for saying this, but when I ask him to work out with me, he always says no."